During the past 10 years nearly 200 human studies and over 100 animal studies have been performed with tritium labelled digoxin. Absorption, excretion, enterohepatic recycling, mechanism of renal excretion, human and dog tissue studies, renal failure and anephric studies, hyper- and hypothyroid states in man and animals, protein binding and studies in infants and children have been accomplished. The ensuing grant period suggests the exploration of abnormal absorption and metabolism as a major area of application with "case finding" simplified by digoxin radioimmunoassay 1; turnover studies to be accomplished with "unusual" serum levels when detected. Compartmental analysis studies are underway which should yield useful information from data already obtained. Radioautographic studies (planned but not yet done) with H3 digoxin of higher specific activity on heart, kidney, liver should be of great interest, as well as specific areas of the conduction system in dogs. Identification of the sites and mechanism of digoxin absorption from the G.I. tract have been initiated and will continue through remainder of G.I. tract. Methods involve column chromatography, thin layer chromatographic identification, digoxin antibodies and Wilzbach labelled H3 digoxin, all previously described.